Roman numerals

It is surprising how frequently roman numerals crop up, whether in the preface to a book, a trendy t-shirt or to write dates.

Latin or Roman numerals use the following digits: I, V, X, L, C, D and M and these correspond to 1, 5, 10, 50, 100, 500 and 1000.

There is no zero, an omission which limits their usefulness.

There are three basic rules required to decipher roman numerals.

Rule 1. When digits are repeated, they are added together.

Rule 2. When a smaller digit occurs before a larger digit, it is subtracted from the larger digit.

Rule 3. When a larger digit occurs before a smaller digit, the digits are added together.

In cases where either Rule 2 or Rule 3 could apply, Rule 2 takes precedence over Rule 3.

The Roman numerals. One to ten

Remember the first rule. A repeated digit is added to the preceding digit.

I is 1
II is 1+1 = 2
III is 1+1+1 = 3
IIII is 1+1+1+1 = 4

Four is more usually written as IV. Where a smaller digit (here I) occurs before a larger digit (here V), it is subtracted. (Rule 2). This results in : V - I, which is 4. The roman numeral V is equivalent to five.

V is 5

VI is 6. A smaller numeral is added to a larger numeral when it precedes the larger. So VI is 5 + 1 = 6.

VII is 7
VIII is 8

IX is 9. (Following the rule stated above as the roman numeral X is equivalent to ten).

These rules mean that the same roman numerals can be written in more than one way. An example is the number 28 could be written as XXVIII 10+10+5+1+1+1 or XX1X 10+10+10-1.

Examples

The same rules are applied to larger numbers. So, for the roman numeral XL, X, ten, occurs before L, fifty, giving L - X which is 50 - 10 = 40.

Roman Numerals. Bigger numbers

The roman numeral for 50 is L.

The roman numeral for 100 is C.

The roman numeral for 200 is CC.

The roman numeral for 300 is CCC.

The roman numeral for 400 is CCCC.

The above numbers all follow the normal rule which is to add repeated digits. The roman numeral for 500 is D. This can also be written by a character which looks like a backwards C preceded by a I.

The roman numeral for 1000 is M.

The roman numeral for 2000 is MM.

The roman numerals from 1-100 or I to C

one I
two II
three III
four IV (or IIII)
five V
six VI
seven VII
eight VIII
nine IX
ten X
eleven XI
twelve XII
thirteen XIII
fourteen XIV
fifteen XV
sixteen XVI
seventeen XVII
eighteen XVIII
nineteen XIX
twenty XX
twenty one XXI
twenty two XXII
twenty three XXIII
twenty four XXIV
twenty five XXV
twenty six XXVI
thirty XXX
forty XL
fifty L
sixty LX
seventy LXX
eighty LXXX
ninety XC
one hundred C